There are two types of tyre pressure monitoring systems (TPMS) currently available which alert a vehicle's driver to abnormal tyre pressure conditions: direct measurement systems and indirect measurement systems. A direct measurement system measures tyre pressure directly with physical pressure sensors. Indirect measurement systems measure something other than actual tyre pressure, such as relative wheel angular velocities or axle to road height changes.
The class of direct measurement systems can be further categorized according to the means employed to provide power to the sensor and implement the communication channel between a vehicle's wheel mounted sensor means and chassis mounted receiving means. Notwithstanding power supply difficulties, just getting the signals off electrical sensors that are mounted inside the rotating wheels presents a serious problem. The prior art approach typically involved wheel mounted, battery powered radio frequency (RF) transmitter modules which continue to suffer from system limitations and reliability problems.
Limitations of battery powered RF transmitter modules include:                a dependence on battery power sources inside the tyre (Once batteries are depleted, operation is compromised and replacement is costly);        in order to conserve battery power, continuous monitoring is not possible, (Transmission of sensed information takes place only when a pressure activation floor is passed (non-deterministic transmission algorithm));        difficulties identifying tyre positions after tyres undergo rotation schedules (Usually, tyre positions must be manually reprogrammed);        cross-talk between other adjacent vehicles with active transmitters and receivers;        the receiver's electronics being subject to deafening by spurious energy fields from external sources (e.g., television transmitters, garage door openers and CB radios etc.,); and        being affected adversely by the influence of metallic vehicle parts on energy field density patterns in the vicinity of the receiver antennae.        
The performance of the aforementioned prior art approach varies greatly between vehicle models because every vehicle model has a different geometry that may interfere adversely with energy field densities.
In contrast, the present invention is based on the direct measurement TPMS approach which employs a two-wire communication channel between wheel mounted sensors and chassis mounted receiver electronics. Importantly, this approach obviates the need for sensor batteries inside the tyre, can use the minimum number of physical conductors (two), is economical and has a highly predictable and reliable performance.
The object of the present invention is to provide an economical and highly reliable system to alert the driver of a vehicle to any abnormal pressures, temperatures, angular velocities or force vectors which may exist in any or all of the vehicle's tyres (including the spare tyre) under all driving or stationary conditions. The system is deterministic and is not affected by tyre replacement, tyre rotations, or deafening of the receiver electronics by radio frequency interference.